Fluorene- and fluorenone-based molecules as electron-transporting SAMs for photovoltaic devices

New semiconductors containing fluorene or fluorenone central fragments along with phosphonic acid anchoring groups were synthesized and investigated as electron transporting materials for possible application in photovoltaic devices. These derivatives demonstrate good thermal stability and suitable electrochemical properties for effective electron transport from perovskite, Sb2S3 and Sb2Se3 absorber layers. Self-assembled fluorene and fluorenone electron-transporting materials have shown improved substrate wettability, indicating bond formation between monolayer-forming compounds and the ITO, TiO2, Sb2S3, or Sb2Se3 surface. Additionally, investigated materials have compatible energetic band alignment and can passivate perovskite interface defects, which makes them interesting candidates for application in the n-i-p structure perovskite solar cell.

Contact angles of water droplets on TiO 2 , Sb 2 S 3 , or Sb 2 Se 3 substrates and substrates coated with fluorene and fluorenone derivatives monolayer were estimated using system DSA25B from Kruess GmbH and analysis software "Drop Shape Analysis" version 1.1, while the contact angle of water and perovskite precursor droplets on non-modified and modified ITO samples were measured by an optical goniometer (The Ossila).Measurement range 5-180°; Measurement accuracy ±1°.The geometry optimization DFT calculations of compounds 2, 4, 7, 11 were performed using TURBOMOLE version 7.0 software [1], with Becke's three parameter functional, B3LYP [2,3], and def2-SVP [4,5] basis set in vacuum.The surface morphology of thin films were carried out in a dual-beam system of a scanning electron microscope Helios Nanolab 650 equipped with an EDX spectrometer from Oxford Instruments.Was using 2keV voltage for ITO/perovskite and ITO/ETMs/perovskite samples.A few nanometers of chromium were sputtered on the cross-section due to the non-conductive/ low conductivity surface.
Samples were placed in a chamber up to 2E -5 mbar pressure.

Cyclic voltammetry measurement
Cyclic voltammetry (CV) measurements were carried out by a three-electrode assembly cell from Bio-Logic SP-150.The measurements were carried out at a glassy carbon electrode in DMF solutions containing 0.1 M tetrabutylammonium hexafluorophosphate as an electrolyte, Pt as the counter electrode and as the reference electrode.Each measurement was calibrated with ferrocene (Fc) and potentials calculated to Fc + /Fc.Conversion factors: ferrocene in DMF vs SCE 0.45, SCE vs SHE: 0.244, SHE vs vacuum: 4.43 [6,7].The half-wave potential was  1/2   obtained as an average value between maxima of cathodic and anodic potentials: . The optical band gaps was estimated from the  1/2   = 1/2

Sample preparation for energy level measurements
For the ionization energy level measurements, the samples were made on indium tin oxide (ITO) covered glass (Präzisions Glas and Optik GmbH, 20 Ω/sq.).Samples for photoconductivity measurements were made on slide glasses with the Al electrodes deposited on them.The Al electrodes were deposited using thermal evaporation in the vacuum method and their thickness was 100 nm.The distance between Al electrodes was 2mm.The studied materials were dissolved in methanol creating a solution with a concentration of 1 mg/ml.The films were formed by the drop-casting method, keeping the glass substrate on a hotplate at 80°C temperature.The film preparation was done in Ar glovebox.The thickness of the samples was in the range between 400 nm and 500 nm.The monolayer samples were made by dissolving the studied materials in methanol with the concentration of 0.5 mg/ml.The layer was formed by spin-coating method.
The rotation speed was 2000 rpm, the acceleration was 2000 rpm/s, and the rotation time was 60s.The samples were dried on a hotplate at 80°C temperature for 15 minutes.Afterwards the excess material was rinsed off using methanol.

Time-resolved photoluminescence (TRPL) measurement
To investigate the fluorescence decay kinetics on the microsecond time were deposited by ultrasonic spray pyrolysis in air from a solution of SbCl 3 (99,999 % ThermoScientific) and SC(NH 2 ) 2 (99,99 %), Sb/S molar ratio 1:3, dissolved in methanol (99.8 %).The 1M stock solution from SbCl 3 in methanol was prepared inside a glovebox with controlled humidity (<7 ppm).The precursor solutions were prepared in room conditions.
Ultrasonically nebulized precursor solution was transported to the preheated to 185 °C glass/ITO/TiO 2 substrates with helps of carrier gas (compressed air with flow rate of 2.5 L/min).
The Sb 2 S 3 thin films were crystallized by rapid annealing approach at 270 ℃ for 6 min in N 2 atmosphere.[8,9].Sb 2 Se 3 absorber layers were deposited from a Sb 2 Se 3 powder (5 N, Sigma-Aldrich) using Vapour Transport Deposition (VTD) setup under vacuum level of 10

Preparation of ITO/ETM/perovskite substrates for Photoluminescence Spectroscopy and TRPL measurement
ITO glass substrates were cleaned for 15 minutes in an ultrasonic bath with 2 % Mucasol solution at ~ 40 °C.After rinsing with water, water and acetone, the substrates were further cleaned by annealing at 250 °C for 30 minutes and treated in an UV-ozone cleaner for 15 minutes.60 µl of the ETM (0.5 mg/ml in methanol) solution was dropped uniformly on the ITO substrate and after 20 seconds, the sample was spin-coated at 2000 rpm for 30 seconds.
Afterwards, the sample was annealed at 100 °C for 10 minutes, left for 1 minute to cool down, washed with 1 ml of pure methanol and substrate was dried under a stream of argon gas.Then, 60 µl of perovskite solution was spin-coated at 1000 rpm for 10 seconds, followed by 6000 rpm for 25 seconds.10 seconds before the end of the spin-coating, 250 µl of chlorobenzene were dropped.The samples were annealed at 100 °C for 1 hour.

Preparation of Glass/perovskite substrate for Photoluminescence Spectroscopy and TRPL measurement
Corning © glass substrate was cleaned with 2 % Mucasol solution in water and after rinsing with water, water and acetone, the substrate was further cleaned by annealing at 500 °C for 30 minutes.Then, 80 µl of perovskite solution were spin-coated at 1000 rpm for 10 seconds, followed by 6000 rpm for 25 seconds.10 seconds before the end of the spin-coating, 500 µl of chlorobenzene were dropped.The sample was annealed at 100 °C for 1 hour.

Preparation of ITO/ETM substrates for contact angle measurement
ITO glass substrates were cleaned for 15 minutes in an ultrasonic bath with 2 % Mucasol solution at ~ 40 °C.After rinsing with water, water and acetone, the substrates were further cleaned by annealing at 250 °C for 30 minutes and treated in an UV-ozone cleaner for 15 minutes.60 µl of the ETM (0.5 mg/ml in methanol) solution was dropped uniformly on the ITO substrate and after 20 seconds, the sample was spin-coated at 2000 rpm for 30 seconds.Then, the samples were annealed at 100 °C for 10 minutes, left for 1 minute to cool down, washed with 1 ml of pure methanol and substrates were dried under a stream of argon gas.

Preparation of Sb 2 S 3 and Sb 2 Se 3 thin films
min in air to assure the formation of the anatase phase.Amorphous layers of Sb 2 S 3

of Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 precursor solution
−5mbar.A 30 nm thick Sb 2 Se 3 seed layer was first deposited at 280 °C substrate temperature and 400 °C source temperature.Next, Sb 2 Se 3 film with ~1.1 µm thickness was deposited at 460 °C substrate temperature and 500 °C source temperature for 5 minutes.